update-ref --stdin: narrow scope of err strbuf

update-ref --stdin: narrow scope of err strbuf

Making the strbuf local in each function that needs to print errors
saves the reader from having to think about action at a distance,
such as

 * errors piling up and being concatenated with no newline between
   them
 * errors unhandled in one function, to be later handled in another
 * concurrency issues, if this code starts using threads some day

No functional change intended.

Signed-off-by: Jonathan Nieder <redacted>
Reviewed-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>